In recent years attention has been given to the monitoring of systems in order to provide an indication of impending degradation or failure in order that remedial action may be made before actual failure occurs. The general problems are the selection of an appropriate parameter of performance or condition and the selection of an appropriate technique for monitoring changes in that parameter.
Power supplies are a ubiquitous part of any electrical or electronic system. They condition an input to supply electrical energy to a load. Historically, power supplies were of a linear type utilising electronic components operating in their linear regions. However, this technology is known to be inefficient. Linear power supply technology, although still used, has largely been superseded by switching mode power supply systems, in which power is dissipated for the most part only when solid state switches such as transistors are transitioning between their ‘on’ and ‘off’ states. Such systems are therefore very efficient when compared to ‘linear’ power supplies. Typically, the switching frequencies are high (50 kHz to 1 Mhz), allowing the use of small components that result in a very compact power supply.
In subsea systems the ‘topside’ power supply is often designed as an uninterruptible power supply (UPS), wherein an input supply is conditioned and used to charge batteries. The batteries then supply power to a switching regulator that conditions the supply into a suitable form for use subsea. Switching mode power supplies may also be used in subsea power supplies to convert the umbilical supply into lower voltages for subsea electronic modules and, for example, the operation of actuators for valves.
Historically, faults in capacitors have been the primary cause of power supply failures. Even with improved manufacturing processes, capacitors continue to cause failures. The lifetime of a capacitor is dependent on voltage, current, and temperature stresses to which it is subjected during its service life. These effects are substantially greater in power supplies than in low-voltage signal processing circuits. As a capacitor ages, its series resistance will tend to increase, resulting in localised heating and thence to localised arcing, that ultimately is liable to cause breakdown of a metallized film dielectric. This dielectric breakdown will result in a ‘hard’ short circuit or open circuit condition and capacitor failure. However, before failure occurs, increase in the series resistance affects the performance of the capacitor and thereby in turn will affect the performance of the output filter of the switching mode power supply.
Switching power supplies have carefully designed filters that remove as much of the switching frequency from the output as is practically possible. If the performance of the filter reduces because of a capacitor's degradation, more of the switching frequency will appear in the power supply's output voltage than is specified by the manufacturer. Therefore, as the power supply ages, so the amplitude of the switching frequency in the output will increase.
State Of The Art
It is known from US2008/018195 A1 to measure noise in a power supply circuit by generating a sine wave of variable frequency and amplitude and cross-correlating this signal with the noise from the power supply. Other documents which employ cross-correlation in different contexts are U.S. Pat. Nos. 4,430,611 and 6,424,138-B1, which describe spectrum analysers, the document US2004/0206170 A1, which describes the detection of torsional vibrations, and the document EP-1533624-A1, which describes cross-correlation between a test signal and an output signal of an electrical circuit.